PROJECT SUMMARY Novel influenza D virus (IDV) utilizes bovines as a primary reservoir with periodical spillover to other mammalian hosts including pigs and horses. Of greater public health importance, serological evidence of IDV infections in humans has been demonstrated. Preliminary investigations have revealed that IDV binds to human 9-O-acetylated N-acetylneuraminic acid (Neu5,9Ac2), the receptor of human influenza C virus (ICV), and non-human N-glycolylneuraminic acid (Neu5Gc9Ac) and likely utilizes them for viral entry. Further functional studies indicate that IDV is more efficient in recognizing both human Neu5,9Ac2- and non-human Neu5Gc9Ac-containing glycans than ICV, and ICV seems to preferentially bind to human Neu5,9Ac2 over non-human Neu5Gc9Ac. In agreement with this, ICV has a limited host range with humans as a reservoir. These results suggest that IDV and ICV diverge in communicating with sialic acids (SA) for infection. The hypothesis is that subtle differences in the sequence and structure of the receptor-binding pocket residing in the hemagglutinin-esterase-fusion (HEF) protein between IDV and ICV cause substantial differences in the virus-receptor interaction, which can expand or reduce, or change tissue and species tropisms. Furthermore, the recent observation on the extracellular resistance of IDV to low pH inactivation versus the intracellular requirement of low pH for viral entry suggests a novel entry/fusion mechanism by which IDV employs to deliver its genome into the target cell. Three aims are proposed to address these hypotheses in this R01 application, which involve a comparative study of IDV and ICV. Aim 1 will elucidate the entry mechanism of IDV, while Aim 2 will define the molecular basis of the HEF receptor binding affinity and specificity and its impact on IDV tropism. Aim 3 is deigned to address how two IDV lineages, swine and bovine, read the glycan receptors differentially and how this information is transduced to different tissue and species tropisms between these two groups of IDVs. In summary, this proposal leverages the team's expertise in IDV research and novel aspects of IDV-receptor interaction and entry to elucidate the cross-species transmission mechanism of IDVs. Understanding molecular details of virus-receptor interaction may lead to development of novel strategies for the control and prevention of IDV and ICV infections in humans.